1ST year official and after watching the college games on saturday, the following question came to mind.
Runner crosses the neutral zone and is grapsed by more than one player of B. The pile up continues as the pile moves slowly downfield (forward progress not stopped). Lineman of A comes charging into the back of the pile and pile moes forward 2-3 yards and everyone goes down. Rather a common sight during any game. I am wondering why there is never a flag for either (1) assisting the runner by lineman of A. Or (2) blocking in the back by (A).
Seems to me of all the times I see this, there is never a flag for either.
NFHS(preferred)/NCAA rulings are welcome.

I wouldn't call aiding the runner as the blocker is conceivably "blocking" and not aiding the runner. The block in the back is another thing, but you are right it is seldom if ever called...

Good question and one that my crew of rookies approached me with this past week...

after discussing with my U (27 yr veteran and NCAA R) this is what we came up with...
1) The "assist" by A is generally called only when A is in the act of pulling or carrying the runner. In the case you provide, the action by A usually results in the pile collapsing, therefore A gains no advantage.

2) The block in the back is ignored using the advantage/disadvantage philosophy stated in 1 above.

Other regions may have different views and I would like to hear them...

I believe the issue with a block in the back is one of two things. Most importantly is a safety issue. All the players in the "moving pile" are basically defenseless. To me, this should be flagged for this reason alone. Secondly wouldbe if the blocking A player dislodges a player of B and the runner is able to continue moving forward, this would be an advantage gained.

I agree that assisting the runner might be a "stretch" of the rule, but at the same time the rule (NFHS) does state that the runner may not be pushed. If the blocking A player hits (pushes) the runner and he goes forward, especially if the line to gain is passed, there is also an advantage gained.

Again, I am more of the opinion that blocking in the back is the predominate candidate for a flag in htis situation.

I've seen "assisting the runner" called on this once or twice --- but only if the blocker is pushing JUST the runner, and not a defender at all.

Regarding the Block in the Back
 

The way I've usually seen this happen, the blocker ends up blocking a teammate... So even though there's a block in the back, there is no foul...